Closes #3414
Closes #3412

Summary: This adds a new event `onSearchBarCancelPressed` that fires
when the user presses the cancel button on the search bar in the
navigation bar. `onSearchBarUpdated` does indirectly fire when the user
presses cancel, as the query will be reset to an empty string, but it is
not possible to distinguish those events from the user simply resetting
the query. My use case for this is to have a view that appears on top of
the normal content of the view controller when the search bar is
focused.

Test Plan: ran `npm run test-js` and confirmed added JS unit tests
passed.

I also tested this with my app, and confirmed the
`onSearchBarCancelPressed` method was called when I pressed the cancel
button.

* test

* Added jest coverage reporter html for Jenkins

RNN's ReactNativeHost implementation prevented users from adding MainReactPackage
themselves, this forced users that wanted to use a custom MainReactPackage to extend
NavigationReactNativeHost which is a bit of an overkill for this simple use case.

closes #3387

There was an issue where ReactRootView was not unmounted if it was destroyed when it was hidden.

This change is introduced so BottomTabs are actually rendered when created
This commit also fixes back button issue where it wasn't properly added when pushing screens.

BottomTabs are all mounted when created, selecting a tab only changes visibility

* Refactored backButton api

* Move StackController to separate package

Implement BackButtonOptions

* Somewhat simplify StackController creation in tests

* Pull backButton logic from StackController

* Handle all TopBar button options in TopBarButtons

* fix back button on iOS

This should fix conversion issues found in RN 0.54